1. Field of the Invention
The present invention relates to a focal point position detector used in an apparatus for recording and reproducing optical information such as an optical disk drive unit. The present invention also relates to an apparatus for recording and reproducing optical information and having an optical pickup of a waveguide type.
2. Description of the Related Art
Recently, an optical recording medium such as an optical disk has been widely used as a medium for recording an image signal, a sound signal, etc. A signal is recorded on this optical recording medium in the shapes of a pit and a difference in reflectivity, etc. and is read by an optical pickup. This optical pickup detects existence and nonexistence of the above pit by irradiating light such as a laser beam onto a surface of the optical recording medium and detecting an intensity level of light reflected on the recording medium.
Such an optical pickup reads recording information as mentioned above and has a function for detecting a tracking error. In this tracking error detecting function, the optical pickup detects whether the irradiating position of a light beam for detecting a pit is shifted onto a right-hand or left hand side from a tracking center of a series of predetermined pits. The optical pickup further has a function for detecting a focusing error. In this focusing error detecting function, the optical pickup detects whether a focal point of the above light beam is located near or distant from a reflecting face of the optical recording medium.
Namely, tracking and focusing controls are performed to correctly irradiate the light beam onto a predetermined track such that tracking and focusing error detecting signals are canceled. Further, the tracking and focusing error signals are used to correctly focus the light beam on the reflecting face of the optical recording medium. A push-pull method, a heterodyne method, a time difference detecting method, etc. are generally known as a tracking error detecting method. An astigmatic method, critical angle detecting method, a Foucault method, etc. are known as a focusing error detecting method.
To fulfill the above functions, the general optical pickup has a beam splitter for separating a light beam reflected on the optical recording medium from a light beam irradiated toward this medium. The general optical pickup also has a lens for converging this reflected light beam in the vicinity of a photodetector such as a photodiode. Further, the general optical pickup has a small optical element such as a prism for detecting tracking and focusing errors.
However, precise processing is required with respect to such a small optical element. Further, it is complicated to adjust mutual positions of the constructional elements when the optical pickup is assembled. Accordingly, the optical pickup using such optical elements is inevitably expensive. Further, such an optical pickup is large-sized and heavy in weight so that it is not advantageous to make a reader compact and light in weight and reduce an access time.
To solve the above problems, there are various kinds of general trials for simplifying the construction of the optical pickup by using a special optical element such as an aspherical lens. However, such an optical element is particularly expensive. Accordingly, cost of the optical pickup using such a special optical element is approximately similar to that of the above general optical pickup although the structure of the optical pickup is simplified.
To solve such problems, Japanese Patent Application Laying Open (KOKAI) No. 63-61430 proposes an optical pickup having one optical waveguide element provided with a convergent diffraction grating. This optical waveguide element functions as the beam splitter, the lens and the prism mentioned above.
Japanese Patent Application Laying Open (KOKAI) No. 63-61430 shows an apparatus for recording and reproducing optical information such as an optical disk drive unit. In such an apparatus, an optical signal is recorded and reproduced from an optical recording medium by the optical pickup. Further, this optical pickup also detects a focal point error signal and a tracking error signal.
In the above optical pickup, a light beam coupled to an optical waveguide layer is converged by convergent grating couplers. Accordingly, the position of a focal point of the light beam is greatly changed in accordance with a change in wavelength of light from a light source so that it is impossible to detect the focal point error signal. Further, the convergent grating couplers function as both a waveguide coupler and a convergent optical system for a waveguide so that a degree of freedom with respect to the arrangement of an optical system is small.